A turret mooring device is known from WO-A-87/06555. The known turret mooring devices consist of a vessel carrying a turret vertically positioned within the hull or in front of the bow, wherein the turret is connected to the hull of the vessel by means of an upper and a lower bearing arrangement, the lower bearing arrangement being situated below water level, the turret being provided with means for fastening and guiding anchor lines and with at least one fluid line extending downwardly through the turret.
The known lower bearing arrangement comprises radial sliding bearings which are mounted on a lower turret unit and on a lower bearing ring that is placed against the bottom plates of the hull. Due to their concept, such bearings permit only a very slight displacement of bearing members relative to each other.
In case of mooring a vessel to a sea bed by the known turret mooring device, the combination of an upper and a lower contact bearing arrangement causes in the turret body both a high rotational torque due to rolling or sliding resistance of the lower bearing arrangement, in case the clearance between the lower bearing pads is relatively small. This may lead to a mooring force due to the dimensions and directions of loads exerted upon the vessel by the anchor lines.
In practice, the high rotational torque can achieve such a dimension, that the bearing arrangement is inhibited from rotating, thus causing twisting of the flexible riser system located underneath the turret relative to the vessel. Such twisting can cause failure of the riser system and thereby spillage of oil out of the lines extending between a wellhead and the vessel.
Sometimes, in turret mooring devices the lower bearing arrangement is omitted for preventing the occurrence of such heavy torques and forces. Obviously, in this case the upper bearing arrangement has a more severe task whereby both the bearing arrangement and its supporting means have to be designed to much higher forces, if it can be designed at all.
Moreover, known lower bearing arrangements are located below water level. Thus, service and exchanging of parts is hard to do.
The object of the invention is to provide a lower bearing arrangement for a turret mooring device which reduces the negative effects of a one bearing arrangement and gives the benefits of a double bearing arrangement to a great extent, wherein easy service and exchange of parts is possible.